In manufacturing a single crystal in accordance with the Czochralski method, a technique for controlling the diameter of the single crystal to be formed and for controlling the shape of the solid-liquid interface is very important to form the single crystal with the required stability while suppressing the generation of crystal defects such as dislocations, the formation of polycrystals or twin crystals. Methods and apparatus employing a member for carrying out such control in a crucible containing raw material molten solution for pulling the single crystal by means of this member have been developed for the Czochralski method.
For example, Japanese Patent Laying-Open Nos. 57-7897 and 61-63596 disclose apparatus in which a molding body having an opening at the center is provided above the raw material molten solution. In these apparatus, the shape of the pulled crystal is controlled as the single crystal is pulled through the opening of the molding body.
Japanese Patent Laying-Open No. 62-288193 discloses a method in which a molding body having an inverted conical shape is dipped in the raw material molten solution contained in the crucible and the single crystal is pulled from the raw material molten solution flowing in the molding body. In said method, by moving the molding body and the crucible relative to each other, the cross-sectional area of the super cooled part of the molten solution formed in the molding body is changed. By adjusting the cross-section in steps for forming a shoulder portion, a cylindrical body and a tail portion are formed and a rapid growth of the crystal can be suppressed.
The inventors of the present invention have studied a method and apparatus in which a coracle having a communicating hole provided at the bottom floats on the raw material molten solution for pulling a single crystal from the raw material molten solution flowing through the communicating hole into the coracle. The coracle is used in the following manner, for example, in order to control the diameter and shape of the single crystal to be grown. Referring to FIG. 1(a), raw material molten solution 45 and liquid encapsulant 47 are contained in a crucible 43, and a coracle 46 is dipped therein. The coracle 46 is adjusted to have an appropriate specific gravity, so that it floats on the raw material molten solution. The raw material molten solution flows through a communicating hole 46a into the floating coracle 46. The surface area of the raw material molten solution in the coracle has an appropriate diameter. Referring to FIG. 1(b), an upper shaft 48 is lowered, and a seed crystal 42 provided at the lower end of the shaft is dipped into the raw material molten solution in the coracle 46. At this time, the temperature of the raw material molten solution is adjusted by means of a heater 44 provided around the crucible 43. Then, referring to FIG. 1(c), the upper shaft 48 is slowly elevated and a single crystal 10 is pulled.
The above described method and apparatus have been proposed to enable a stable growth of the single crystal. However, when a crystal is to be pulled by using a small temperature gradient in the direction of pulling, or when a crystal having a relatively low thermal conductivity is to be pulled, it is often difficult to manufacture such a crystal with a low dislocation density by the above described methods and apparatus.
In the above described method or apparatus, emission of heat from the raw material molten solution is considered to be a major factor causing a higher dislocation density. In order to suppress heat radiation, various methods or measures have been proposed. For example, Japanese Utility Model Laying Open No. 60-172772 discloses a crystal pulling apparatus in which at least one heat intercepting plate is provided in the longitudinal direction of the shaft for pulling the crystal, in order to suppress heat convection from the raw material molten solution. Japanese Patent Laying-Open No. 60-81089 discloses a method in which an elongate crucible is used and a crystal is pulled while the raw material molten solution is covered by the sidewall of the crucible and a reflector for reflecting heat radiation is provided on a crystal pulling shaft.
Further, Japanese Patent Laying-Open No. 60-118699 discloses an apparatus including a member for suppressing heat radiation and heat convection from the raw material molten solution, through which member the crystal pulling shaft is provided above the crucible.
The conventional apparatus or method employing a member for suppressing radiation or convection enables manufacturing of a crystal with a low dislocation density. However, when the crystal is to be pulled with a temperature gradient that becomes smaller in the pulling direction, or when a crystal having a relatively low thermal conductivity is to be pulled, it was often difficult by such method or apparatus to suppress a rapid growth of the crystal especially at the start of the pulling. A rapid crystal growth causes the formation of twins or polycrystals and makes it difficult to manufacture a single crystal with a high reproducibility. Especially when a crystal of a material having a low thermal conductivity such as CdTe is to be pulled, it was difficult to pull the single crystal with a high reproducibility, because of the rapid growth at the start of the pulling.
When a crystal of GaAs or CdTe is to be pulled, the emission of heat from the raw material molten solution is an important cause of the rapid growth of the crystal while pulling takes place. The conventional apparatus and method could not sufficiently suppress the emission of heat during pulling the crystal. Most of the emission of heat is considered to be caused by heat radiation. Therefore, an effective interception of radiation was necessary to effectively suppress the heat emission. If the radiation is intercepted nearer to the raw material molten solution, heat emission can be more effectively suppressed. However, the mechanism for intercepting radiation at an upper portion of the crucible provided in the conventional apparatus is not effective enough, and it was necessary to intercept radiation near the raw material molten solution. Interception of the radiation is considered to be not sufficient even in the method or apparatus in which a shaft for pulling the crystal is lowered so as to bring the radiation intercepting member closer to the molten material when the prior art is trying to improve the suppression of the heat emission.